Disclosed herein is a method of producing hydrogenated amorphous semiconductor materials which are characterized by a low density of defect states in the energy gaps thereof and good electrical, optical and microstructural properties. As used herein, "density of defect states" is defined as the number of defect states, or deep electronic trap sites such as dangling, broken or non-optimally coordinated bonds per unit volume in the band gap of a given material.
Amorphous semiconductor alloy material, such as amorphous silicon alloy material and amorphous germanium alloy material, have previously been produced by glow discharge plasma deposition, chemical vapor deposition, evaporation and sputtering processes. While these processes will be referred to in greater detail hereinafter, it is sufficient for purposes of the instant discussion to note that the amorphous semiconductor alloy materials so produced have possessed a relatively high density of defect states in the energy gaps thereof. Since a significant industrial use of amorphous silicon and germanium alloy materials is in the production of semiconductor devices such as photovoltaic cells, it is important that the alloys used to fabricate the photovoltaic cells provide, inter alia, good electrical transport properties. However, amorphous alloy materials having a high density of defect states in the energy gaps thereof are characterized by a large number of deep electronic trap sites which result in low drift mobility and short lifetimes for the recombination of free carriers, particularly minority carriers. Amorphous semiconductor materials are extremely attractive alternatives to crystalline semiconductors because they (1) can produce as large area devices, and (2) possess a direct band gap with a high optical absorption coefficient as compared to the indirect band gap and low optical absorption of corresponding crystalline materials. It is therefore extremely important for the density of defect states in the band gap of amorphous semiconductor alloy material to be reduced in order for amorphous semiconductors to compete in a cost effective manner with their crystalline counterparts.
Whereas it is now possible to provide the wider band gap amorphous semiconductor material with an acceptably low, although not optimum, density of defect states, it is the narrow band gap material (about 1.4 eV and below) which have yet to be produced with a sufficiently low density of defect states to make more than 13.7% efficient photovoltaic cells. It is believed the reason better narrow band gap materials have not as yet been produced is because (1) the environment in which these low band gap amorphous semiconductor alloy materials have been deposited included a great variety of unwanted species, and (2) the previous methods of combining the precursor materials did not effectively optimize tetrahedral coordination of the deposited material. The numerous species produced by, as well as the parameters involved with, plasma deposition, evaporation, sputtering etc. are inherently uncontrollable, and therefore these processes are unable to provide the extremely high efficiency photovoltaic cells which are theoretically possible.
Since the lowest energy state of amorphous solids such as silicon and germanium occurs if those solids recombine in crystalline form, the aforementioned methods of depositing amorphous semiconductor alloy material resulted in the deposition of a low density, porous amorphous material that contained a great number of bonding vacancies and micro voids. In order to further improve amorphous semiconductor alloy material, it is therefore necessary to modify known methods of deposition so as to either (1) reduce the number of those vacancies and voids, or (2) make these vacancies and voids become a helpful part of the process. Only by improving the number of vacancies and voids can a high density, low defect density material characterized by good transport properties be produced.
Before proceeding further, note that amorphicity is a generic term referring only to a lack of long range periodicity. In order to understand amorphous semiconductor materials, the stoichiometry of the material, the type of chemical bonding, the number of bonds generated by the local order (its coordination), and the influence of the entire local environment upon the resulting bonding configurations incorporated in the amorphous solid must be considered. Amorphous semiconductor materials, rather than being viewed as a random packing of atoms characterized as hard spheres, should be though of as composed of an interactive matrix influenced by electronic configurations and interactions. If, however, one is able to outwit the normal relaxations of the depositing amorphous semiconductor alloy material and utilize the available three-dimensional freedom present in the amorphous state, entirely new and significantly improved amorphous materials may be fabricated. Of course, this creation of new amorphous semiconductor material requires the use of new processing techniques.
The inescapable conclusion to be derived from the foregoing is that by properly coordinating process parameters, it should be possible to create deviant, but desirable, electronic configurations in amorphous semiconductor materials. In order to understand this concept, it must be realized that amorphous materials have several different bonding configurations as available energetic options. For instance, elemental amorphous silicon, although normally tetrahedrally coordinated, has some atoms which are not tetrahedrally bound. Local order is always specific and coexists in several configurations. Stearic and isomeric considerations are involved both with factors influencing amorphicity and with those creating defects in the materials. The constraints in amorphous semiconductor materials are involved with asymmetrical, spatial and energetic relationships of atoms permitted by the varying three dimensional chemical and geometrical possibilities afforded by an amorphous solid. In such a solid there is not only a spectrum of bonding configurations which spans from metallic to ionic, but a spectrum of bonding strengths. A major factor involved in the spectrum of bond strengths is the competitive force of the chemical environment which acts to influence and alter the bond energy. Based on the foregoing, it should be apparent that a greater number of weaker bonds are presented in an amorphous semiconductor material than in its crystalline counterpart.
There exists an important energetic process that leads a material toward amorphicity. More specifically, the preferred chemical bonding configurations of atoms and the field produced by non-bonding electrons can provide the molecular structure with a proclivity toward assuming a more ordered noncrystalline state. The geometries or shapes of these structures are complex, distorted ones formed by localized pressures, repulsions and attractions of surrounding forces. These forces cause compression in one area, elongation in a second, twisting in a third, all in contradistinction to a perfectly repetitive crystalline cell. It can therefore be appreciated that the energetic considerations necessary to complete four-fold coordination depend upon the ability to spatially and energetically mate bonding positions. This is the reason that amorphous silicon (being tetrahedrally coordinated) has a greater number of dangling bonds, weakened bonds and voids than its crystalline counterparts.
In amorphous semiconductor alloy material the normal equilibrium bonding can be disturbed by creating new bonding configurations through the addition of a compensating element or elements with multi-orbital possibilities. For instance, alloying permits the optical band gap to be tailored for specific applications and yet permits chemical modification or doping by reducing defect states in the gap, thereby affecting electrical conduction. The elimination of the crystalline constraints permits a greater variety of bonding and antibonding orbital relationships than are present in a crystalline solid and represents a key element in the synthesis of improved amorphous semiconductor alloy materials.
In summation, the charge carrier transport properties of amorphous semiconductor alloy materials are directly related to deviant bonding configurations due to under or over coordination. It is the chemistry and microstructure existing in amorphous semiconductor materials that permits the utilization of the "super halogenicity" of fluorine for its ability to organize and expand coordination which provides for the design of atomic and molecular configurations best suited for specific purposes.
In other words, it is possible to synthesize and independently control all relevant characteristics of amorphous semiconductor alloy material such as optical band gap, electrical activation energy, melting temperature, hopping conduction, and even thermal conductivity. However, this presupposes that the proper manufacturing techniques are employed. The implementation of proper manufacturing techniques requires a departure from convention. It is just such novel processing technology for synthesizing improved amorphous semiconductor material with optimum characteristics to which one aspect of the instant application is directed.
In contrast to indirect gap crystalline silicon, amorphous silicon alloy materials are a direct gap semiconductor and, therefore, only a material thickness of one micron or less is necessary to absorb the same amount of sunlight as 50 micron thick crystalline silicon. Further, amorphous semiconductor alloy material can be made faster, easier and in larger areas than can crystal silicon alloys, thereby reducing assembly time and cost. While a considerable effort has been made to develop processes for readily depositing amorphous semiconductor alloys or films, each of which can encompass relatively large areas, limited only by the size of the deposition equipment, and which can be readily doped to form p-type and n-type materials where p-n junction devices are to be made therefrom equivalent to those produced by their crystalline counterparts. For many years such work was substantially unproductive. Amorphous silicon or germanium (Group IV) films are normally four-fold coordinated and were found to have microvoids and dangling bonds and other defects which produce a high density of localized states in the energy gap thereof. The presence of a high density of localized states in the energy gap of amorphous semiconductor films results in a low degree of photoconductivity and short carrier lifetime, making such films unsuitable for photoresponsive applications. Additionally, such films cannot be successfully doped or otherwise modified to shift the Fermi level close to the conduction or valence bands, making them unsuitable for making p-n junctions for solar cell and current control device applications.
To date, the reduction of the localized states was accomplished by glow discharge deposition of amorphous silicon alloy films wherein a gas of silane (SiH.sub.4) is passed through a reaction tube for decomposition by an r.f. glow discharge and deposition onto a substrate at a substrate temperature of about 250.degree. C. The material so deposited on the substrate is an intrinsic material consisting of silicon and hydrogen. To produce a doped amorphous material, a gas of phosphine (PH.sub.3) for n-type conduction or a gas of diborane (B.sub.2 H.sub.6) for p-type conduction is premixed with the silane gas and passed through the glow discharge reaction chamber under the same operating conditions. The gaseous concentration of the dopants used was between about 5.times.10.sup.-6 and 10.sup.-2 parts per volume.
D. I. Jones, W. E. Spear, P. G. LeComber, S. Li, and R. Martins worked on preparing a-Ge:H from GeH.sub.4 using similar glow discharge plasma deposition techniques. However, the material obtained gave evidence of a high density of localized states in the energy gap thereof. Although the material could be doped, the efficiency was substantially reduced from that obtainable with a-Si:H. In this work reported in Philosophical Magazine B, Vol. 39, p. 147 (1979) the authors conclude that because of the large density of gap states the material obtained is ". . . a less attractive material than a-Si for doping experiments and possible applications." Further work by Li et al. on a-Ge:H by glow discharge plasma deposition reported in Materials Research Society Symposium Proceedings, Volume 149, pages 187-192 (1989) additionally showed that glow discharge amorphous germanium is susceptible to atmospheric contamination, especially at low substrate deposition temperatures.
Research has also been conducted to evaluate the sputter deposition of amorphous silicon alloy films in an atmosphere of argon (required by the sputtering deposition process) and molecular hydrogen, to determine the results of such molecular hydrogen on the characteristics of the deposited amorphous silicon alloy film. This research indicated that the molecular hydrogen indeed acted as an altering agent which bonded in such a way as to reduce the density of localized defect states in the energy gap. However, the reduction of states achieved by the sputter deposition process was much less than achieved by the RFCVD process described above. P and n dopant gases also were introduced in the sputtering process to produce p and n doped materials. The resultant materials possessed a lower doping efficiency than the materials produced in the n.f. glow discharge process. However, neither the RFCVD nor the sputtering deposition techniques of depositing amorphous semiconductor layers provided n-doped and p-doped materials with sufficiently high acceptor concentration to produce commercial p-n or p-i-n junction devices. While the n-doping efficiency was below acceptable commercial levels, the p-doping efficiency was particularly unacceptable since the width of the band gap was reduced and the number of localized states in the band gap was increased.
Further research was conducted in an attempt to additionally reduce defect states in amorphous silicon alloy materials or in amorphous silicon:hydrogen alloy material. Fluorine was found to readily diffuse into and bond to the amorphous silicon alloy material, substantially reducing the density of localized states therein, because of its electro-negativity and the small size of the fluorine atoms which enables them to be readily introduced into the amorphous silicon host matrix. Fluorine was found to bond to the dangling bonds of the silicon in a manner which is more stable and efficient than is possible when hydrogen is used. However, fluorine introduced into amorphous germanium alloy material or amorphous germanium:hydrogen alloy material, has not, up to the date of the instant invention, produced a narrow band gap material with as low a density of defect states as amorphous silicon alloy material.
Numerous attempts have been made to construct both natural and synthetic crystalline analog materials by special layering techniques with the aim of extending the range of desirable properties which were heretofore limited by the availability of natural crystalline materials. One such attempt involved compositional modulation by molecular beam epitaxy (MBE) deposition on single crystal substrates. Esaki, Ludeke and Tsu, in U.S. Pat. No. 3,626,257, describe the fabrication of monolayer semiconductors by one MBE technique. These modulated prior art structures are typically called "superlattices". Superlattice fabrication techniques are based on the concept that discrete layers may be made to form a one-dimensional periodic potential by periodic variation of (1) alloy composition or (2) impurity density. Typically, the largest period in these superlattices is on the order of a few hundred Angstroms, however, monatomic layered superlattice structures have also been constructed. The superlattices can be characterized by (1) several layers of a material "A" (such as GaAs), followed by (2) several layers of a material "B" (such as AlAs), in a repetitive manner, (3) formed on a single crystal substrate. The optimum superlattice is a single crystal synthetic material with good crystalline quality and electron mean free paths greater than the period. Conventional superlattice concepts have been utilized for special electronic and optical effects; however, because the lattice constants must be very carefully matched, the utilization of these superlattices has been limited.
In related work, Dingle, et al., see U.S. Pat. No. 4,261,771, disclose quasi-superlattices and non-superlattice structures. The former are comprised of epitaxially grown crystalline islands of a foreign material in an otherwise homogeneous layered background material. The latter, non-superlattice structures, are an extension of quasi-superlattice materials in that the islands are grown into columns which extend vertically through the homogeneous layered background material.
In addition to MBE superlattice construction techniques, other researchers have developed layered synthetic microstructures which utilize different forms of vapor deposition, including diode sputtering, magnetron sputtering and standard multisource evaporation and organo-metallic vapor deposition. These materials can be thought of as synthetic crystals or crystal analogues in which the long range periodicity, the repetition of a particular combination of layers, or the grading of layer spacing must be closely maintained. Consequently, superlattice structures, so constructed, are both structurally and chemically homogeneous in the x-y plane, but periodic in the z direction.
In addition to the synthetic material-producing techniques described above, compositionally varied materials and processes for their production are disclosed in copending U.S. Pat. No. 4,520,039 to Stanford R. Ovshinsky, assigned to the assignee of the instant application and the disclosure of which is incorporated herein by reference.
Other methods of producing amorphous semiconductor alloy materials, specially adapted for photovoltaic applications, are disclosed in Assignee's U.S. Pat. Nos. 4,217,374; 4,226,898 and 4,342,044, the disclosures of which are also incorporated hereinto by reference. The deposition techniques described therein are adapted to produce materials including germanium, tin, fluorine and hydrogen as well as silicon. The materials are produced by vapor and plasma activated deposition processes. Further, tandem multiple photovoltaic cell structures are disclosed in U.S. Pat. Nos. 4,891,074 and 4,954,182 assigned to the assignee of the instant application and hereby incorporated by reference.
U.S. Pat. No. 4,569,697 (assigned to the assignee of the instant invention and the disclosure of which is incorporated by reference) is directed to a post deposition diffusion process in which unadulterated amorphous semiconductor material was deposited onto a substrate at a low substrate temperature before the, preferably activated, density of states reducing element was introduced into an ultrahigh vacuum environment. Since (1) an environment was provided in which there were no contaminants present to occupy the available bonding sites of the amorphous semiconductor alloy material, and (2) the deposition occurred at a low temperature, the deposited amorphous semiconductor alloy material was a porous mass of voids, vacancies, dangling bonds, etc. Therefore, the density of states reducing element readily diffused into and was greedily accepted by the deposited material and served to reduce the density of defect states in those regions of the heterogeneous amorphous alloy characterized by a relatively low density of defect states. An annealing step completed the diffusion of the density of states reducing element through the deposited material and served to reduce the density of defect states in those regions of the heterogeneous amorphous alloy characterized by a relatively high density of defect states. Finally, a strain relieving element was introduced to relax bonding stresses in the resultant alloy and ion implantation was employed to reduce the density of defect states in regions of the alloy which were unaccessible to the diffusion process.
Another method which has been used to produce amorphous semiconductor materials is laser ablation. Laser ablation essentially consists of striking a target of the desired semiconductor material with a laser beam to evaporate material therefrom. The evaporated material is deposited onto a substrate which is disposed adjacent to the target. In the past, laser ablation has been used as a laboratory curiosity to fabricate amorphous silicon, amorphous germanium, and hydrogenated amorphous silicon alloy material. For further information on laser ablation deposition processes in general see Sankur, et al "Formation of Dielectric and Semiconductor Thin Films by Laser-Assisted Evaporation", Applied Physics A, Vol. 47, pages 271-84, 1988, the disclosure of which is incorporated herein by reference.
The laser ablation of a-Si and a-Si:H are disclosed by Hanabusa, et al in "Dynamics of laser-induced vaporization for ultrafast deposition of amorphous silicon films", Applied Physics Letters, Vol. 38, pages 385-7, March 1981 and "Reactive laser-evaporation for hydrogenated amorphous silicon", Applied Physics Letters, Vol. 39, pages 431-2, September 1981, the disclosure of each is incorporated herein by reference. In the former, the production of a-Si is disclosed; however, the material is not of high enough quality for use in photovoltaics due to the lack of hydrogen present in the host matrix thereof. The material produced by the latter publication does include hydrogen therein; however, the majority of the incorporated hydrogen is in the dihydride form and therefore the microstructural and photoelectrical properties are poor.
Production of a-Ge by laser ablation is disclosed by Sankur, et al in "High-quality optical and epitaxial Ge films formed by laser evaporation", Journal of Applied Physics, Vol. 65, pages 2475-8, March 1989 and by Afonso, et al in "Good-quality Ge films grown by excimer laser deposition", Applied Surface Science, Vol. 46, pages 249-53, 1990, the disclosure of each of which is incorporated herein by reference. However, as with the a-Si films described above, the materials produced by these disclosed methods are not of photovoltaic quality due, inter alia, to the absence of hydrogen compensation.
There still exists a long felt need for a high quality narrow band gap material to make full use of the infrared (IR) part of the solar spectrum in photovoltaic applications. Glow discharge deposited a-SiGe.sub.x ;H alloy has been used for the bottom cell of tandem or triple junction solar cells, to absorb the IR light from the sun. The optoelectronic properties of glow discharge a-SiGe.sub.x :H, however, degrade rapidly once its band-gap is decreased below 1.4 eV. It is believed that in a-SiGe.sub.x :H film, H does not bond favorably to Ge (is not fully coordinated) and therefore, for narrower band gap alloy material, i.e., more Ge content, the material possesses an increased number of uncompensated defects.